1. Field of the Invention
The invention relates, generally, to electron-beam (e-beam) cure of epoxy compositions and specifically to deep-section, e-beam cure of herein disclosed epoxy monomers.
2. Relevant Art Discussion
Today, the use of electron beam radiation to carry out the rapid, pollution free, low energy polymerization of multifunctional vinyl monomers is a well established technique. E-beam curing, as it is called, is most widely applied to the cure of coating materials which are either heavily pigmented or are comparatively thick in crossection, such as cable insulation. In such applications, ultraviolet radiation-induced polymerization is impractical whereas the great depth of penetration of the electron radiation permits the polymerization of these substrates with relative ease. E-beam curing is most widely applied to multifunctional acrylate and methacrylate monomers and polymerization results by a free radical mechanism initiated by solvated electrons and free radicals produced by bond scission in the monomers. the use of multifunctional acrylate and methacrylate monomers for e-beam curing suffers from the fact that these materials are costly, toxic and require an inert atmosphere for their proper cure. For high speed applications, the cost of nitrogen used as an inert atmosphere may render this process economically disadvantageous. Acrylate and methacrylate based monomers also may not have the requisite properties to meet demanding applications. One such application lies in the area of composites fabrication. In this application, excellent thermal resistance, adhesion to the fiber reinforcements and superior mechanical properties of the cured resin is required. These properties have not been obtained using acrylate and methacrylate monomers.
There are also cases known in which the cationic polymerization of certain monomers can take place under the influence of electron beam radiation. These polymerizations, which proceed by the generation of cation-radicals and free ions (cations), require highly purified and dry monomers. For this reason, such polymerizations are not suitable for use in practical applications. Polymers bearing pendant epoxide groups have been used as negative tone electron beam photoresists. In this case only a few percent (1-5%) reaction of the epoxy groups is sufficient to effectively crosslink the resin.
Several workers have recently disclosed in patents the use of cationically polymerizable systems in e-beam curing. The first of these, Covington et al., U.S. Pat. No. 4,657,844, described the use of polyvinyl formal and polyvinyl carbazole, together with onium salts, as curable systems for microlithographic applications. In another patent, U.S. Pat. No. 4,654,379, Lapin describes the use of vinyl ethers in combination with cellulose esters to form interpenetrating network polymers at doses from 0.1 to 10 Mrads. Onium salts are also required in this technology. Still another patent, Banks et al., U.S. Pat. No. 4,849,320, describes the e-beam cure of mixtures of cationically and free radically curable resins in the presence of onium salts for microlithographic applications. Cycloaliphatic epoxides are mentioned in this patent.
Recently, the .gamma.-ray and electron beam induced cationic polymerization of vinyl ether monomers and oligomers in the presence of reducible onium salt photoinitiators has been reported. While the e-beam induced cationic polymerization of vinyl ethers is interesting, the use of these materials would only be appropriate for rather low performance applications. Accordingly, my work has been focused on the e-beam polymerization of epoxy resins. Cured epoxy resins are well known for their excellent chemical resistance, mechanical properties and adhesion to various substrates. However, initial attempts to use e-beam irradiation for the polymerization of commercially available bisphenol-A diglycidyl ether monomers or the difunctional cycloaliphatic epoxide monomer, 3,4-epoxycyclohexylmethyl-3', 4'-epoxycyclohexane carboxylate, were disappointing. Polymerization occurred slowly in the presence of diaryliodonium and triarylsulfonium salts but only after massive radiation does (100-300 Mrad.) were applied. These radiation doses are too high for consideration for any practical application. However, it was determined that there would be considerable commercial and academic interest, if a means for curing epoxy resins using e-beam irradiation could be developed. Epoxy resins are currently used in many applications such as composites, where a combination of excellent high temperature and mechanical properties is essential. However, currently, no practical examples of e-beam curable epoxy monomers are known.